A known mobile telephony system includes geographic cells, each having a respective base station and being connected to a switching centre. Each base station has access to a given number of channels, which may be both frequency shared and time shared. The cells are collected in large cell groups, in which adjacent cells are assigned separate frequencies, so-called fixed channel assignment. All frequencies of the system are found within a cell group. Several cell groups are placed adjacent one another, in order to cover wide geographic regions and the frequency assignment in the cells is selected so that mutually separate groups will not disturb one another. A detailed description of the aforesaid system is found in CMS 88, Cellular Mobile Telephone System, Ericsson Telecom AB, 1988. Adaptive mobile telephony systems are also known, in which each base station has access to all system channels. A mobile subscriber is assigned a channel subsequent to measuring occurrent disturbances, so-called dynamic channel assignment. A system of this kind is intended for external use, normally by vehicular subscribers. Both the base station and the subscriber mobile station has a relatively high transmission power. Corresponding mobile telephony systems are known for indoor use, for instance in factories or offices, for instance as described in the 39th IEEE Vehicular Technology Conference, Volume 1, May 1-3, 1989, Dag .ANG.kerberg: "Properties of a TDMA Pico Cellular Office Communication System". This report describes an adaptive telephony system comprising cordless, portable subscriber apparatuses which can be connected to a subscriber exchange through base stations. One requirement is that such indoor systems shall have a large connection coupling capacity. Accordingly, the system has a relatively large number of small cells each with a respective base station, and both the base stations and the portable subscriber apparatuses have a relatively low signal strength. This enables the same channels to be utilized repeatedly within a building without the occurrence of troublesome disturbances.
One problem encountered in mobile telephony is to offer the subscribers a sufficiently large number of channels within a limited frequency range. This frequency range is assigned by the authorities and may not be exceeded such that adjoining frequency ranges will be disturbed. Frequency time-sharing enables the number of channels within a frequency range to be increased considerably, although the number of channels remains restricted. Consequently, it is difficult to have an indoor system and an outdoor system for mobile telephony of large capacity within the same geographical region. Particular difficulties are found in introducing an indoor system in a region in which an outdoor system has already been established. It is also difficult for a subscriber to utilize both systems with one and the same mobile station.